Add separate button for advanced binding

[AbstractQbit]

[InfinityGhost]

Sizing on GTK isn't enough now, suggesting this change.

diff --git a/OpenTabletDriver.UX/Controls/BindingEditor.cs b/OpenTabletDriver.UX/Controls/BindingEditor.cs
index c6f605e..0b8dfb7 100644
--- a/OpenTabletDriver.UX/Controls/BindingEditor.cs
+++ b/OpenTabletDriver.UX/Controls/BindingEditor.cs
@@ -28,7 +28,8 @@ namespace OpenTabletDriver.UX.Controls
 
             var tipButton = new BindingDisplay(App.Settings?.TipButton)
             {
-                Width = 250
+                MinimumSize = new Size(300, -1)
             };
             tipButton.BindingUpdated += (sender, binding) => App.Settings.TipButton = binding;
 
@@ -54,7 +55,8 @@ namespace OpenTabletDriver.UX.Controls
             {
                 var penBinding = new BindingDisplay(App.Settings?.PenButtons[i])
                 {
-                    Width = 250,
+                    MinimumSize = new Size(300, -1),
                     Tag = i
                 };
                 penBinding.BindingUpdated += (sender, binding) =>
@@ -85,7 +87,8 @@ namespace OpenTabletDriver.UX.Controls
             {
                 var auxBinding = new BindingDisplay(App.Settings?.AuxButtons[i])
                 {
-                    Width = 250,
+                    MinimumSize = new Size(300, -1),
                     Tag = i
                 };
                 auxBinding.BindingUpdated += (sender, Binding) =>

[AbstractQbit]

Set y to 0 instead of -1 because it was crashing on Windows with System.InvalidOperationException: Layout measurement override of element 'Eto.Wpf.Forms.EtoBorder' should not return NaN values as its DesiredSize. exception.

[InfinityGhost]

Waiting on #650 and we'll resolve conflicts here if they appear. We'll deal with that later, in v0.5.x